Biogas is an important source of renewable energy. Biogas can be produced from organic matter by digestion e.g. at waste water treatment plants, waste treatment plants, and agricultural site anaerobic plants. It can also be collected from landfill sites. Thus, the term “biogas” is used in this document as a common term to refer to any of “biogas”, “landfill gas”, “digester gas”, etc. The main components of biogas are methane CH4 and carbon dioxide CO2, and it typically also comprises small amounts of hydrogen sulphide H2S, moisture, and siloxanes as impurities. The use of biogas as an energy source is based on energy-releasing combustion of the components thereof. Biogas is typically used as a fuel in gas engines, gas turbines, micro-turbines, and fuel cells producing electricity. It can be also be combusted to produce heat.
Siloxanes are semi-volatile organosilicon compounds, which are used in a number of industrial applications and in consumer products, such as cosmetics and lubricants. As a result of their wide use, a substantial amount of siloxanes ends up in landfills and sewage, where they volatilize into landfill gas or digester gas. Siloxanes in biogas are usually organosilicon compounds. A siloxane is a functional group in organosilicon chemistry with the Si—O—Si linkage.
The increasing interest in the production of biogas and renewable energy in waste management and sewage treatment has created significant concern about the presence of siloxanes in biogas. Siloxanes as gaseous compounds are not reactive or corrosive as such, but they form hard, abrasive silica as a deposit on various surfaces of the equipment wherein biogas is used. Such deposit also acts as a thermal and electrical insulator. This deposit can cause serious damages, such as fouling, corrosion, and lower energy output in the biogas utilization equipment.
Table 1 lists some siloxanes most commonly occurring in digester gas. Based on their molecular structure, siloxanes are commonly divided into linear (denoted with “L”) and cyclic (denoted with “D”) siloxanes. The siloxane concentration of biogas is generally in a range of 0-50 mg/m3, typically below 10 mg/m3.
TABLE 1BoilingCompoundAbbreviationpoint, ° C.HexamethyldisiloxaneL2107OctamethyltrisiloxaneL3153DecamethyltetrasiloxaneL4194DodecamethylpentasiloxaneL5245HexamethylcyclotrisiloxaneD3135OctamethylcyclotetrasiloxaneD4176DecamethylcyclopentasiloxaneD5211DodecamethylcyclohexasiloxaneD6245TrimethylsilanolTMS99
According to the literature, the most common siloxanes in a landfill gas are D3, D4, D5, L2 and L3. In addition to siloxanes, the landfill gas also contains relatively large concentrations of silanol.
To reduce the above harmful effects of siloxanes, the siloxane content of biogas should be kept or made as low as possible. For example, biogas production plants typically comprise biogas purification systems where the detrimental siloxanes are removed from biogas by using various methods. Thereby, the siloxane content is lowered before the use of biogas as fuel e.g. in gas engines. The siloxane content of biogas may, however, vary at different locations and even at the same location during different periods. Therefore, continuous monitoring of the siloxane content before and/or after the purification system would be needed in order to optimize the operation of the siloxane removal equipment when it's cleaning effect changes or the siloxane content of the gas to be cleaned changes.
Injecting biogas to a natural gas pipeline is another example of applications where reliable siloxane content monitoring is required. Strict requirements are set for the quality of the biogas to be injected to the natural gas pipeline, necessitating accurate determination of the siloxane content in the biogas.
The need for monitoring siloxanes in gases is also recognized in plastics manufacture and semiconductor production industries where siloxane monitoring is needed especially in controlling the air quality in a clean room.
Traditionally, siloxane content in biogas is determined offline by taking a gas sample, which is analyzed in a laboratory. The methods used in sampling and laboratory analysis are slow and laborious, and there is a danger of losses in connection with sampling and storage. The methods in use today do not allow on-site monitoring and direct monitoring of the process equipment. The most common analysis methods are based on a combination of gas chromatography and mass spectrometry (GC/MS). When the siloxane contents of gases to be analyzed are as low as 0.1-5 ppm, high demands are made on the analysis method.
Spectroscopic analysis methods exist which are based on determining a complete IR spectrum of a sample and which allow a comprehensive analysis of the composition of biogas. By means of multicomponent analyzers base on e.g. FTIR (Fourier Transform Infrared) spectrometry, it is in principle possible to determine the concentrations of all significant gas components. An FTIR analyzer is, however, very expensive and complicated apparatus. Furthermore, its use as an online measurement device and the evaluation of the measurement results require long experience.
US 2010/0223015 A1 discloses a method for monitoring siloxane compounds in a biogas by FTIR spectrometry. A first absorption spectrum is generated based on a ratio of a first spectral measurement and a second spectral measurement. The first spectral measurement is from a non-absorptive gas having substantially no infrared absorption in a specified wavelength range of interest. The second spectral measurement is from a sample gas comprising the biogas. The method also includes the step of calculating a concentration of at least one siloxane compound in the biogas using a second absorption spectrum based on, at least, a first individual absorption spectrum for a known concentration of the at least one siloxane compound. The measurement equipment is complicated and expensive.
As a more simple and low-cost approach, JP 2006098387 A discloses an analyzer for measuring siloxane content of a gas by NDIR (Non-Dispersive Infrared) technique. The analyzer comprises a broadband IR source and an optical filter restricting the light interacting with a sample gas to be analyzed and a reference gas to a wave number range of 1250-770 cm−1. Various siloxanes have their absorbance maxima within this wave number range, so the presence of siloxanes in the sample gas can be determined on the basis of detected absorption. However, in this wave number range the measurement result is also affected by many other components of biogas, such as moisture, carbon dioxide, methane, etc, the concentrations thereof being orders of magnitude higher than that of the siloxanes. Therefore, specific measures are needed to eliminate the influence of interfering gas components. This makes the analyzer complicated and the interpretation of measurement results challenging. For instance, a dehumidifier or a moisture analyzer is needed to eliminate the effect of moisture on the measurements. A specific detector system is used in the analyzer, which detects light in said wave number range of 1250-770 cm−1 
To summarize, there is a continuous demand in the market for technology enabling reliable, online analysis of siloxane content in gases, in particular in biogas, with reasonable costs.